Pipes used to carry liquids and gases commonly transport all types of materials including water, natural gas, solid and liquid sewage, as well as various other accumulations from the pipe. Currently, during initial installation, sewer pipes must be scarified first in order to provide a clean and profiled surface for the subsequent coating with such materials as polyvinyl chloride sheet. Older sewer pipes eventually need to be scarified to remove contamination, corrosion and loose material. The term “scarify” is intended to mean a removal of a layer or portion of the interior surface of the pipe whether or not it is contaminated or corroded in order to provide a profile to that surface. The term “profile” refers to a surface having peaks and valleys giving it a granulated texture similar to that of sandpaper. Such a surface provides a greater surface area so as to maximize bonding of the coating material. Profiled surfaces can range from fine-grained similar to the granularity of #150 Fine Sandpaper to a coarse-grained similar to the granularity of #60 Coarse Sandpaper.
The interior surface of a pipeline carrying solids, liquids and gases generally degrades over time as the pipe walls interact chemically and physically with the substances flowing through them. In particular, a sewer system's interior walls corrode and deteriorate because corrosive materials contaminate the surface degrading the metal and concrete used to build the sewer. The corrosive material arises from both the sewage and from the wastewater itself, and also from the digestive by-products of bacteria found in the sewage, which proliferate in the anaerobic environment. The corrosion causes the walls of the sewer pipe to physically decay, eventually reducing their overall thickness.
The principal source of corrosion is sulfuric acid, which arises as a product of the materials transported in a sewer pipe and the sewer environment itself. Various metal sulfates found in the sewage quickly convert into hydrogen sulfide by: reducing to sulfide ions in the waste water, combining with hydrogen in the water and out gassing above the liquid as hydrogen sulfide gas. Additional hydrogen sulfide originates from bacteria containing contaminants, which accumulate on the relatively rough concrete below the maximum liquid level. Bacteria found in these accumulations thrive in the anaerobic sewer environment producing hydrogen sulfide gas as a respiratory bi-product. Oxygen from the liquid below and oxygen condensing from the water in the air react with the hydrogen sulfide on the pipeline walls creating the highly corrosive sulfuric acid. The sulfuric acid attacks the calcium hydroxide in the concrete sewer walls leaving calcium sulfates, which ultimately crumble and fall off of the interior of the wall substantially reducing its thickness.
The waste water level in a sewer varies over the course of a 24-hour period. The flow is at its lowest level between 1:00 AM and 6:00 AM in the morning but it rises distinctly in the daytime and the pipe may operate near capacity. Because of the gaseous nature of the hydrogen sulfide, the pipe walls are predominately corroded in the portions of the wall above the minimum liquid level. Portions of the walls which are always below the water level are not subjected to such high concentrations of hydrogen sulfide gas or sulfuric acid and consequently do not experience the same levels of decay.
Eventually the sewer walls must be restored or they can suffer permanent damage leading to great expense. The restoration process is a two-step operation that consists of first cleaning all of the contaminants (and possibly outer layers of corrupted concrete) from the surface of the pipe and then applying a protective coating over the newly cleaned pipe surface. Attempting to apply a protective coating without first cleaning the pipe surface is futile because it does not stop the decay that has already begun underneath the coating. Furthermore, the protective coating itself does not adhere well to the contaminated surface. Thus, cleaning is an essential element of the restoration process. Even new sewer pipe must be scarified to provide a profiled surface and to remove the laitance and any dirt, oil or grease.
As previously mentioned, a sewer system typically operates at high capacity during the day with decreasing flow overnight. In order to restore the sewer pipes without diverting the flow (a costly and sometimes impossible alternative), a bulk of the work must be done at night during the brief period when the flow is at a minimum. As previously outlined, the restoration process involves both profiling the pipe surface and then applying a protective coat. In practice, the rate of restoration is impaired because manual scarifying (i.e., grinding) takes a proportionally greater amount of time than does the application of the protective coat. Consequently, a need exists for an automated scarifying process. Such a process will improve the rate of cleaning of the pipeline's interior walls making restoration without diversion a cost-effective possibility. Further, automation of the process can help to ensure that the same intensity of cleaning is applied to the entire surface without the quality variation that is inherent in manual execution.